A bomb blast at an opposition protest march in Bangkok wounded at least 28 people, emergency officials said, in the latest unrest during a "shutdown" of the capital aimed at toppling the government. 

"The bomb went off shortly after 1pm," said the deputy national police spokesman, Anucha Romyanan.

Police were investigating what type of explosive device caused the blast. Television footage showed several people lying on the ground as ambulances took away wounded.

Protesters were seen searching nearby buildings for the attackers.

Prime Minister Yingluck Shinawatra has faced more than two months of street demonstrations aimed at forcing her elected government from office and installing an appointed body to oversee loosely defined reforms such as an end  to alleged vote buying.

Eight people have been killed and hundreds injured in street violence since the protests began.

"Yingluck must take responsibility," one of the protest leaders, Satit Wonghnongtaey, said on stage soon after the blast.

The protesters aim to curb the political dominance of Yingluck's billionaire brother, fugitive former prime minister Thaksin Shinawatra, whom they accuse of controlling the government from self-exile.

Source: Agencies